Drop keyword arguments to read().

Currently, requests-testadapter fails on any version of requests after 1.2.0, because in 1.2.1, requests began sending a keyword argument to the `BytesIO.read` method, assuming they'll always get a urllib3 object (which isn't what requests sends).

This pull requests accepts and then drops that keyword argument. This also takes the unit tests, which were failing on requests >= 1.2.1, back to passing (edit: well, once I get rid of Python 2 bugs I created :-)).

If accepted, this pull request will fix #1.